A constant-velocity universal joint includes an outer joint member having outer ball grooves, an inner joint member having inner ball grooves, torque transmitting balls which are disposed to be guided respectively by a pair of the outer ball groove and the inner ball groove, and a ball cage containing the torque transmitting balls. Such a constant-velocity universal joint may be interposed between a transmission and a driving wheel to perform a function of transmitting torque.
The ball grooves of the constant-velocity universal joint may be formed by joining portions having different shapes so as to have double shapes and to have a high articulation angle (e.g., more than 50 degrees).
In a conventional constant-velocity universal joint of high-articulation angle, a ratio O/H of an offset value O which is a distance between an inflection point of an outer ball groove and a line connecting centers of the torque transmitting balls in a state that an outer joint member and an inner joint member form 0 angle and a distance H between a center line of an inner joint member and a center of a ball in a state that an outer joint member and an inner joint member form 0 angle is generally within a range of 0.13 to 0.15.
However, in a constant-velocity universal joint of high-articulation angle, a joint having the value of the ratio O/H within this range has a great power loss.